The present invention provides polyamines which contain urea groups, a process for the preparation thereof and their use to produce adhesives, sealants, embedding compounds, moulded items or coatings.
Polyamine/epoxide resin systems are characterised, inter alia, by excellent adhesion to metals, very good resistance to chemicals and outstanding corrosion prevention properties. In the case of solvent-containing formulations and powder coating systems, cross-linked films with high flexibility are obtained by using epoxide resins with high molecular weights and/or polyaminoamides, e.g. those based on dimeric fatty acids, as hardeners. Coatings based on solvent-free liquid resins and solvent-free amino hardeners are brittle due to the low molecular weights of the epoxide resins and the high network density resulting therefrom. Nowadays, therefore, tar substitutes, e.g. coumarone resins, are used for plasticising purposes in solvent-free formulations. These types of coating tend to become brittle in the long term, in particular when using relatively large amounts of hydrocarbon resins, due to migration of the non-functional constituents.
Good and permanent elastification of epoxide resins can be achieved by combining them with polyurethanes. Thus, e.g. in DE-A 23 38 256, high molecular weight amine-terminated polyetherurethane ureas are prepared by reacting prepolymers which contain free isocyanate groups with amines in highly dilute solution and then curing the product with epoxide resins.
The use of the, in particular aromatic, solvents required for this is a disadvantage in practice, both from an industrial and also a physiological point of view. On the other hand, the viscosity of the solvent-free reaction products, such as those prepared in accordance with DE-A 23 38 256, is too high for use in practice.
DE-A 24 18 041 describes a process for preparing elasticised moulded articles and two-dimensional structures in which epoxide and amine compounds are reacted which are obtained by hydrolysis of prepolymeric ketimines or enamines. Using this process, thermoset materials which are resistant to chemicals, adhere well and have improved properties are obtained. However, the process described is costly from a technical point of view.
DE-A 21 52 606 describes reactive systems based on alkylphenol-blocked polyisocyanates and polyamines, which may optionally also be cured in combination with epoxide resins. These reactive systems are also associated with some technical disadvantages: on the one hand, the blocking agents being released have comparatively low molecular weight so they migrate out of the coating over the course of time, which can lead to adhesion problems. On the other hand, reactive systems based on alkylphenol-blocked polyisocyanates and polyamines have a relatively high viscosity and the actual mechanical properties of the end products do not satisfy all requirements.
In contrast to this, EP-A 480 139 describes a process for reacting non-blocked NCO prepolymers with polyamines at temperatures of 140–170° C. However, this process can be applied only to prepolymers with aliphatic or cycloaliphatic isocyanate groups due to the very high reactivity of aromatic isocyanate groups when reacting with amines. Since aliphatic and cycloaliphatic isocyanates can be prepared only in a more costly manner (more expensively) than aromatic isocyanates, polyurethanurea amines prepared by this method have hitherto been of no industrial significance.
In order to facilitate targeted reaction of polyisocyanate prepolymers with excess amounts of diamine, it has therefore been suggested on several occasions that the polyisocyanates be used in a blocked form, as described for instance, in CA-A 12 19 986, EP-A 293 110 or EP-A 82 983. These publications disclose using, as preferred blocking agents, phenols or substituted phenols. After completion of reaction with the polyamines, these phenols cannot be removed, or not completely removed, from the reaction mixture by distillation, due to their high boiling point. Retention of these optionally substituted phenols in the mixture or in the plastic materials, however, leads to the disadvantages mentioned above. Furthermore, it is pointed out in these publications that in principle the other conventional blocking agents from polyurethane chemistry may also be used, e.g. oximes, caprolactam, malonates and acetoacetates. However, since none of these blocking agents can be incorporated into the polymer structure during the course of epoxide hardening, these types of compounds are not normally used in traditional amine/epoxide chemistry. The use of such blocking agents instead of the preferably used, optionally substituted, phenols, does not provide significant advantages.
In accordance with EP-A 457 089, on the other hand, secondary amines with preferably low boiling points are used as blocking agents. If these amines are retained in the reaction mixture after de-blocking, slow evaporation of the very pungent compounds (odour pollution) readily takes place. Although in principle the secondary amine can be incorporated into the system when used in epoxide systems, this takes place relatively slowly, especially in the case of applications at low temperatures (e.g. room temperature) so that some of the amines evaporate before reacting. In a particularly preferred application, the amine blocking agent is distilled out of the reaction mixture after de-blocking. Although this procedure leads to products which do not release a gas (odour pollution), it is very costly.
Thus, the object of the invention was to provide elasticising, amine hardener mixtures for epoxides which do not have the disadvantages of the systems in the prior art.
The object can be achieved by the provision of the hardener mixtures according to the invention and the process for preparing them which are described in detail below.